init: avoid ERROR log due to missing SEPolicy.zip

One of the first ERROR messages in logcat of a normal boot of Cuttlefish
is from failure to open SEPolicy.zip.  This condition is expected.
Therefore don't try to load SEPolicy.zip when it doesn't exist.  This
replaces the following log messages:

0     0 I init    : Error: Apex SEPolicy failed signature check
0     0 I init    : Loading APEX Sepolicy from /system/etc/selinux/apex/SEPolicy.zip
0     0 E init    : Failed to open package /system/etc/selinux/apex/SEPolicy.zip: No such file or directory

... with just:

0     0 I init    : No APEX Sepolicy found

Change-Id: If3a77407c35130165df5782b9ef91912e8374dbf
diff --git a/init/selinux.cpp b/init/selinux.cpp
index b6d483a..51093d8 100644
--- a/init/selinux.cpp
+++ b/init/selinux.cpp
@@ -667,10 +667,16 @@
 //
 void PrepareApexSepolicy() {
     // If apex sepolicy zip exists in /metadata/sepolicy, use that, otherwise use version on
-    // /system.
-    auto dir = (access((kSepolicyApexMetadataDir + kSepolicyZip).c_str(), F_OK) == 0)
-                       ? kSepolicyApexMetadataDir
-                       : kSepolicyApexSystemDir;
+    // /system. If neither exists, do nothing.
+    std::string dir;
+    if (access((kSepolicyApexMetadataDir + kSepolicyZip).c_str(), F_OK) == 0) {
+        dir = kSepolicyApexMetadataDir;
+    } else if (access((kSepolicyApexSystemDir + kSepolicyZip).c_str(), F_OK) == 0) {
+        dir = kSepolicyApexSystemDir;
+    } else {
+        LOG(INFO) << "APEX Sepolicy not found";
+        return;
+    }
 
     auto sepolicyVerify = SepolicyVerify(dir);
     if (!sepolicyVerify.ok()) {